The safe and reliable operation of high voltage direct current transmission project is threatened by the problem of deposition on grading electrodes in the inner valve cooling water system. In this paper, the calculation models of single, double and triple grading electrode systems are established in order to analyze the aggregation stage and deposition stage of the discharged particles in inner cooling water. By means of finite element method, the distributing characteristics of the charged particles around the grading electrodes are calculated. Furthermore, the deposition process on the surface of grading electrodes is revealed through simulation calculation according to the electrophoretic deposition principle. The results show that a threshold value of electric current exists on the grading electrodes when capturing the charged particles. In the engineering condition, the capturing rate of the charged particles by single, double triple electrode system is 12.6%, 33.3% and 33.9%, respectively. The morphology and growth characteristics of the deposition by simulation of electrophoretic deposition are in high consistency with the deposition in the engineering practice.